Rapid eye movement (REM) sleep is when we dream — but new research shows it may also be when we forget.

In a mouse study, a team of Japanese and U.S. researchers has found that brains can employ REM sleep to actively forget excess information. The authors also point to a group of neurons deep inside the brain that control this process of forgetting during sleep.

Dreamland

“Ever wonder why we forget many of our dreams?” asks Thomas Kilduff, Ph.D. and a senior author of the study.

“Our results suggest that the firing of a particular group of neurons during REM sleep controls whether the brain remembers new information after a good night’s sleep.”

REM is one of the several sleep stages our bodies cycle through every night. It generally starts around 90 minutes after we fall asleep and gets its name from the rapid, darting movements of our eyes during this phase. It’s also characterized by increased heart rates, immobile limbs, dreaming, and brain wave patterns reminiscent of wakeful states.

The role of sleep in memory storage has been studied in the past — especially its role in helping our brains form new memories. However, researchers haven’t looked into whether it can also help the brain cut out excess information is stored throughout the day. Recent studies in mice have shown that during sleep, REM sleep included, certain synaptic connections involved in learning are selectively ‘pruned’ — which effectively destroys the memories they store.

The present study is the first to investigate how such a process could take place.

“Understanding the role of sleep in forgetting may help researchers better understand a wide range of memory-related diseases like post-traumatic stress disorder and Alzheimer’s,” said Janet He, Ph.D., who is the program director at the National Institute of Neurological Disorders and Stroke (NINDS).

“This study provides the most direct evidence that REM sleep may play a role in how the brain decides which memories to store.”

Dr. Kilduff’s team, together with that of Akihiro Yamanaka, Ph.D., at Nagoya University in Japan, have spent years looking into the role of a hormone called hypocretin/orexin in controlling sleep and narcolepsy. Narcolepsy is a disorder that makes people feel excessively sleepy during the day and sometimes experience changes reminiscent of REM sleep, including loss of muscle tone in the limbs and hallucinations. Narcolepsy could be linked to the loss of the neurons that produce this hormone in the hypothalamus, a peanut-sized area found deep inside the brain.

For the present study, Dr. Kilduff collaborated with members from Hokkaido University in Sapporo, Japan to look at cells neighboring those neurons that secrete melanin-concentrating hormone (MCH), which is involved in the control of appetite and sleep. They found that a majority (52.8%) of hypothalamic MCH cells fired when mice underwent REM sleep, about 35% fired only when the mice were awake, and about 12% fired at both times — this is consistent with previous findings on the subject.

Electrical brain recordings and tracing experiments further revealed that many of the hypothalamic MCH cells sent inhibitory messages to the hippocampus, the brain’s memory center, through long axons.

“From previous studies done in other labs, we already knew that MCH cells were active during REM sleep. After discovering this new circuit, we thought these cells might help the brain store memories,” said Dr. Kilduff.

To test this idea, the researchers turned MCH neurons in mice on and off during memory tests. They were specifically interested in the role these cells play in retention, which is the period between learning something and it being stored (consolidated) into long term memory — a sort of memory “limbo”.

The team reports that activating MCH cells during retention worsened long-term memory consolidation while turning them off improved it. For example, activating the cells reduced the time mice spent sniffing around new objects compared to familiar ones, but turning the cells off had the opposite effect.

Further experiments suggested that the MCH neurons perform this task during REM sleep. Mice performed better on memory tests when MCH neurons were turned off during REM sleep, and turning the neurons off while the mice were awake or in other sleep states had no effect on memory.

“These results suggest that MCH neurons help the brain actively forget new, possibly unimportant information,” said Dr. Kilduff.

“Since dreams are thought to primarily occur during REM sleep, the sleep stage when the MCH cells turn on, activation of these cells may prevent the content of a dream from being stored in the hippocampus — consequently, the dream is quickly forgotten.”

The paper “REM sleep–active MCH neurons are involved in forgetting hippocampus-dependent memories” has been published in the journal Science.